Am I obligated to allow my wife to visit and bid farewell to her brother before his travel, knowing that I swore she wouldn't, or else I would consider her divorced?

If you intend to effectuate the divorce when your wife visits her brother, then her divorce takes place upon her departure. If you have not divorced her twice before, it is recommended that you permit her to visit her brother, then reconcile with her before her waiting period ends by saying: "I have reconciled with my wife," or by engaging in intercourse with her.